Colloidal HgTe nanocrystals with widely tunable narrow band gap energies: From telecommunications to molecular vibrations

Article abstract of DOI:10.1021/ja058440j

A convenient, aqueous-based synthesis of stable HgTe nanocrystals with widely size-tunable room temperature emission between wavelengths of 1.2 to 3.7 μm is demonstrated. By the choice of the thiols, applied as stabilizers, we optimized the growth dynamics, the luminescence quantum yields (up to 40%), and a ligand-exchange procedure, required to transfer the nanocrystals from water to nonpolar organic solvents. The latter is greatly improved and facilitated by the use of mercaptoethylamine as initial stabilizer. The possibility to tune the HgTe nanocrystal sizes from 3 to 12 nm and to control their surface functionalities (hydrophobic and hydrophilic) makes them very promising for the development of infrared optical devices, emitting in the wavelength region between the telecommunications and the molecular vibrations. Copyright

Full text of DOI:10.1021/ja058440j

Published on Web 02/22/2006
Colloidal HgTe Nanocrystals with Widely Tunable Narrow Band Gap
Energies: From Telecommunications to Molecular Vibrations
Maksym V. Kovalenko,* Erich Kaufmann, Dietmar Pachinger, J u¨ rgen Roither, Martin Huber,
Julian Stangl, G u¨ nter Hesser, Friedrich Sch a¨ ffler, and Wolfgang Heiss
Institute of Semiconductor and Solid State Physics, Johannes Kepler UniVersity, Linz A-4040 Linz, Austria
Received December 22, 2005; E-mail: maksym.kovalenko@jku.at
1
2
For the visible and the near-IR spectral region, a multitude of
highly luminescent wide-gap semiconductor nanocrystals (NCs)
have been chemically synthesized. Despite the enormous success
3
of colloidal NCs, used, for example, in lasers, as fluorescent
4
5
markers in biological systems, or in single photon sources, the
development of NCs with tunable IR emission up to wavelengths
6
of more than 3 µm is still challenging. Hg chalcogenides are most
appropriate to obtain narrow band gaps from NCs in the quantum
confinement regime because their bulk constituents exhibit zero
band gaps, whereas those of all alternative binary narrow gap
semiconductors, such as lead chalcogenides and group III antim-
onides, are at least 0.18 eV (InSb, 300 K).7
So far, the best synthetic routes to Hg chalcogenide NCs are
those based on thiol-capping in aqueous medium.8 The optical
characterization of NCs in aqueous solutions is, however, restricted
to the near-IR, due to the opaqueness of water at wavelength >1.3
µm. Synthetic approaches to HgTe10 NCs in the organic solvents
suffered from the high reactivity, instability, and tendency to
uncontrolled growth, even at temperatures as low as 70-100 °C.
Therefore, we have synthesized HgTe NCs in aqueous solutions
and transferred them to organic solvents by a ligand-exchange
procedure. By the choice of the stabilizer, we optimized the growth
dynamics, the luminescence quantum yields (up to 40%), and the
phase transferability. As a result, we obtain stable HgTe NCs, giving
room temperature emission which is strongly size-tunable between
the wavelength of 1.2 and 3.7 µm.
,9
Figure 1. Representative room temperature PL (a) and absorption (b)
spectra of DT-capped HgTe NCs in CCl4. The insets show the size
dependence of the PL peaks with the corresponding quantum efficiencies
The HgTe NCs were prepared in aqueous solutions via a room
temperature reaction between Hg(ClO ) and H Te gas (bubbled
4 2 2
through) in the presence of the hydrophilic thiols, thioglycerol (TG),
thioglycolic acid (TGA), L-cysteine, mercaptoethanol (ME), and
mercaptoethylamine (MEA), as stabilizers (see Supporting Informa-
(a) and illustrate the phase transfer completeness for MEA used as initial
stabilizer (b).
efficiencies (QE) and the same range of PL tuning. As shown in
the inset of Figure 1a, the PL peak wavelength can be shifted from
1.2 to 3.5 µm by increasing the nanocrystal average size from 3 to
12 nm. In particular, the PL peak energy as a function of diameter
2
tion for details). To achieve a continuous generation of H Te gas,
we make use of a specially designed electrochemical cell as a cheap
and convenient source. There, the electrochemical reduction of
elemental Te in acidic solution results in an efficient (current yield
-
1
-2
d (nm) is well approximated by E[eV] )0.3 + 0.2d + 5.7d
.
∼
0.6) generation of H
2
Te gas. To push the emission of HgTe NCs
By comparing the evolution of the PL peaks with growth time, we
observe a much faster growth rate for MEA-capped particles than
for TG-capped ones (Figure S2). For the largest NCs, emission up
to 4 µm is observed, thus the luminescence overlaps with the
molecular vibration band of the stabilizer, shown in Figure 1b. For
these long wavelengths, the quantum efficiency (QE) is found to
be on the order of 0.5-2%, while it increases up to 40% for the
near-IR, similar to that demonstrated for oleic acid-capped PbSe
to longer wavelength, their sizes were increased by a postsynthetic
heat-treatment, typically at 75-80 °C. Subsequently, the ligand
exchange with dodecanethiol (DT), a hydrophobic thiol, was carried
out. The structural characterization and size determination of the
NCs were done by wide-angle X-ray diffraction (XRD) and
transmission electron microscopy (TEM).
For all used stabilizers, the as-synthesized HgTe NCs have almost
identical sizes of 3-4 nm and PL peak wavelength between 1.1
and 1.3 µm. The choice of the initial hydrophilic capping thiol,
however, is crucially important for controlling the growth dynamics
at elevated temperatures, for the transferability from the aqueous
to the organic phase, and for the optical properties of larger HgTe
NCs. All results discussed in the following are restricted to MEA-
and TG-stabilized NCs since they provide the highest PL quantum
6
NCs. The HgTe NCs retain their luminescence properties when
they are incorporated into polymers, such as poly(methyl meth-
acrylate) (Figure S3), which is of high importance for future
applications. A similar tuning behavior as that for the PL is observed
by the linear absorption spectra (Figure 1b). In the near-IR, they
show well-resolved peaks corresponding to the lowest electronic
transitions and a broadening of them with increasing particle size.
3516
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J. AM. CHEM. SOC. 2006, 128, 3516-3517
10.1021/ja058440j CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. XRD patterns of 3.5 and 7.2 nm HgTe NCs. The XRD spectrum
of bulk HgTe11 (coloradoite) is presented for comparison.
(hydrophobic and hydrophilic) of the HgTe NCs together with their
emission in the spectral range from the telecommunications to the
molecular vibrations make them very promising for applications
in infrared optical devices, such as NC-sensitized polymer solar
cells,13 microcavity light emitters, or molecule detection systems.
Figure 2. TEM image of ∼9 nm DT-capped HgTe NCs emitting at 3 µm
a). High-resolution TEM images of single crystal (b) and twined (c)
particles.
(
14
Acknowledgment. Financial support from the Austrian Science
Foundation FWF (START Project Y179 and SFB-IRON) and the
GME is gratefully acknowledged. TEM images were taken at the
Technical Service Unit of Johannes Kepler University.
One important advantage of the MEA-capped HgTe NCs is their
extremely high transferability from the aqueous to the organic phase.
Previously, Gaponik et al.12 have established that an efficient phase
transfer of thiol-capped CdTe NCs needs rather high amounts of
DT and acetone to be added to achieve a large interface area
between the aqueous and the organic phase (moderately stable
emulsion). We observe a very similar behavior for the TG, TGA,
and ME-capped HgTe NCs with transfer efficiencies typically
e80%. In contrast, the phase transfer of MEA-capped HgTe NCs
is completed in time scales of seconds after the injection of a small
amount of DT (as low as 1/8 of the volume of the aqueous phase),
as illustrated in inset of Figure 1b. This enables a facile, virtually
one-pot synthesis of large amounts of hydrophobic HgTe NCs.
Between the transferability and the colloidal stability of aqueous
HgTe NCs, a correlation is observed, indicating that the ligand
exchange is strongly influenced by the binding energy between Hg
atoms at the nanocrystal surface and the thiol (see Supporting
Information).
The structural characterization by TEM and XRD shows the high
crystallinity of the HgTe NCs, as well as their uniformity in size
and regularity in shape (Figure 2). Most HgTe NCs are predomi-
nantly formed as defect-free single crystals (Figure 2b). A few
percent of the NCs, however, show interesting planar defects, such
as twins (Figure 2c) and stacking faults (Figure S4). The heat-
treatment of HgTe NCs increases the occurrence of planar defects
and broadens the size distribution of NCs from ∼10 to ∼30%. The
latter was successfully refined to ∼10% by size-selective precipita-
tion. Like HgTe bulk (coloradoite),12 the NCs have the zinc blende
crystal structure, as is determined by the XRD patterns in Figure
Supporting Information Available: Experimental details, TEM
and SAED data. This material is available free of charge via the Internet
at http://pubs.acs.org.
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4
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